Vestibular organs are provided with a rich efferent innervation, originating in the brain stem. The long-term objective of this study is to understand the function of the efferent vestibular system (EVS) in the processing of vestibular information. There are three specific aims: 1) To determine the effects of electrical activation of different efferent contingents on the several afferent groups in the barbiturate- anesthetized chinchilla. The problem was originally investigated almost 20 years ago. There is a need to update results by taking into account recent findings on the organization on afferent and efferent pathways, as a recent suggestions that EVS stimulation may have more heterogeneous influence on afferent gain than had been suspected. This study provides essential background information concerning the function of the EVS. 2) To study potential long-term or trophic efferent influences by comparing the acute and chronic effects on afferent discharge on eliminating the efferent innervation or the contralateral labyrinth. The presence of neuroactive peptides and metabotrophic receptors, as well as certain features of the response to electrical stimulation suggests that the EVS exerts such an influence, yet the problem has not been previously addressed. 3) To determine the conditions leading to the discharge of efferent neurons and to efferent-mediated changes in afferent transmission. Recordings will be made from vestibular-nerve afferents and from efferent neurons within the brain stem in alert squirrel monkeys free to move their heads in a horizontal plane. For both groups of neurons, responses to active and passive head movements will be compared. The possibility that efferents get inputs from the neck, from other proprioceptors or from several vestibular organs will be explored. Some information on vestibular inputs to the EVS are best obtained in acute preparations; these experiments will be done in decerebrate chinchillas.